Cold Pack Chemistry: Exploring Endothermic and Exothermic Reactions

Background Information for Teachers

Instant cold packs (Figure 1) are "ice packs" that, unlike regular ice or gel packs, do not need to be stored in a freezer. Inside the outer bag, water and another chemical (typically ammonium nitrate) are separated by a second bag or tube. When you squeeze the outer bag, the inner bag breaks, allowing the ammonium nitrate to mix with the water. This results in an endothermic chemical reaction—one which absorbs heat, causing a local decrease in temperature. This is in contrast to an exothermic chemical reaction, which releases heat, causing a local increase in temperature.

Image Credit: Ben Finio, Science Buddies / Science Buddies

an instant cold pack

Figure 1. Two instant cold packs.

Designing an instant ice pack is both a chemistry problem and an engineering problem. There are criteria and constraints around the design of the ice pack. For example, how big should it be? How cold should it get? How cold is too cold? How long will it stay cold? In turn, these questions can be used to inform decisions about which chemicals to use and the corresponding chemical reaction. How should the reaction be activated? What is the proper ratio of reactants? Are the chemicals safe if the bag breaks and they spill? In this lesson plan, your students will explore different chemical reactions as they pose questions like these themselves (and seek answers). We recommend the chemical reactions shown in Table 1, although you could substitute or add others. Results from Science Buddies staff testing of these reactions are show in Figure 2.

Image Credit: Ben Finio, Science Buddies / Science Buddies

This graph shows a plot of minimum or maximum temperature in degrees F versus concentration in g/mL for four different chemical reactions: ammonium chloride and water, calcium chloride and water, magnesium sulfate and water, and baking soda and vinegar. The ammonium chloride graph is concave up with a minimum of 36 degrees at a concentration of 0.3 g/mL. The calcium chloride graph is concave down with a maximum of 182 degrees at a concentration of 0.9 g/mL, however the right-hand side of the curve is truncated because higher concentrations were not tested. The magnesium sulfate graph is concave up with a minimum of 58 degrees at a concentration of 0.7 g/mL. The baking soda graph is very slightly concave up with a minimum of 60 degrees at a concentration of 0.3 g/mL.

Figure 2. Experimental data for maximum/minimum temperature vs. concentration for the chemical reactions shown in Table 1.

Additional Background Links

• The Engineering Design Process, Science Buddies
• Endothermic vs. Exothermic Reactions, Khan Academy

Prep Work (15 minutes)

• Print the student worksheets
• Set out chemicals and equipment for each group

Engage (10 minutes)

1. Optional demo if you have an instant cold pack:
   1. Tell students to handle it gently (no squeezing or bending it), and pass it around the room so they can all feel that it is at room temperature.
   2. Follow the instructions to activate the pack (e.g. by squeezing it) and pass it around the room again.
2. If you do not have a cold pack, you can show your students this video of cold pack activation.
3. Prompt student questions and answers about the cold pack.

   How do you think the cold pack got cold if we never put it in the refrigerator or freezer?

   Explain that the cold pack contains two different substances. When the bag is squeezed, the substances mix and an endothermic chemical reaction occurs. An endothermic reaction absorbs heat from its surroundings, causing the temperature to drop. Other types of chemical reactions, called exothermic reactions, generate heat and cause the surrounding temperature to rise. Even if you do not know what chemicals are in the reaction, you can tell whether it is endothermic or exothermic by observing the temperature change.

4. Explain that in this lesson, students' goal is to design their own instant cold pack. First, they will use the engineering design process to define the criteria and constraints for a "good" cold pack. Then they will explore different chemical reactions and determine which one best fits their criteria and constraints.

Explore (2-4 hours)

Note: depending on your available time, there are two different ways you can approach this lesson.

• Option 1 (4 hours): Each group can decide on their own criteria and constraints for a cold pack. They can then test each chemical reaction listed in Table 2. Based on their own criteria and constraints, each group can then pick the best reaction (and concentration) and present their decision to the rest of the class. You can compare results between groups.
• Option 2 (2 hours): You can agree on criteria and constraints for a cold pack as a class. Each group can test just one chemical reaction, and you can combine your results as a class. You can then select the single best reaction and concentration based on your agreed-upon criteria. Note that this option makes it easier to collect data for multiple trials in a timely manner, if multiple groups do the same reaction.

1. It might be tempting to dive right into doing experiments, but in order to reach a successful solution, first groups will need to define the criteria and constraints of their cold pack. Explain the difference between a criteria and a constraint, and ask students to work in groups to list criteria and constraints on their worksheets:
   1. A criteria is something you want your design to do; some objective or goal it needs to meet. For example, you could require that the cold pack reach a certain temperature, or stay below a certain temperature for a certain amount of time. You could require that the cold pack not exceed a certain size limit—a cold pack that gets very cold, but is too big to carry, would not be very practical!
   2. A constraint is a practical limitation on your design. Many real engineering projects have constraints on the resources available to do the project—for example the amount of time or budget available, the number of people who can work on the project, or the available materials. While there is no "budget" for this project, in this lesson, students may only have certain chemicals available to work with.
2. After each group (or the entire class) has agreed upon criteria and constraints, they should don their safety gear and move on to testing the chemical reactions. The general procedure for doing so is as follows (note that the quantities here assume you are using a 50 mL beaker, see note in materials section). See Table 2 for important safety information.
   1. Measure 2 g of the solid (calcium chloride, ammonium chloride, magnesium sulfate, or baking soda) and pour it into a beaker.
   2. Prepare at least 20 mL of the liquid (water or vinegar) in a separate graduated cylinder or beaker.
   3. Measure and record the initial temperature of the liquid.
   4. Slowly add the liquid to the beaker containing the solid until the total volume in the beaker reaches 20 mL. The bulb of the thermometer should be completely covered when you put it all the way into the beaker. Note that due to the volume of the solid, you may not need to add all of the liquid. This will result in a concentration of 2 g / 20 mL = 0.1 g/mL.
      1. Note: For the baking soda and vinegar reaction, pour very slowly so the bubbles do not spill out of the container. You will need to wait for the bubbles to die down to accurately measure the volume.
   5. Put the thermometer in the beaker. Use the thermometer (or a glass stirring rod) to gently stir the solution while closely watching the temperature. Be careful to avoid splashing. When the temperature reaches a minimum or maximum value (when it stops changing), record this temperature in your data table. Note that if you wait too long, the temperature will start going back down (for exothermic reactions) or back up (for endothermic reactions), so it's important to record the extreme value (the value farthest from room temperature).
   6. Pour the solution down the drain and flush with water.
   7. Rinse and dry the beaker and thermometer (and stirring rod if used).
   8. Repeat steps 2a–2g with 4 g of solid.
   9. Continue repeating this process in 2 g increments until it is no longer practical to do so (e.g. the initial volume of the solid exceeds 20 mL, or there is so little liquid added to reach 20 mL that the result is a sludge instead of a saturated liquid solution). For most of the reactions this will occur somewhere in the 14–18 g range.
3. Analyze the data.
   1. Calculate a change in temperature for each trial (the final temperature minus the initial temperature).
   2. Plot the change in temperature versus the concentration for each chemical reaction tested.

Reflect (15 minutes)

1. Each group should now revisit their criteria and constraints in light of the results of their experiments. Do any of them require revision? For example, did they set a temperature goal that wasn't achieved by any of the chemical reactions? Is there anything they didn't consider initially that they observed in their experiments (e.g. the foaming/bubbling nature of the baking soda + vinegar reaction probably means it wouldn't be a good choice for a cold pack, but students might not have thought of this initially).
2. Based on their data and revised criteria and constraints, each group (or the entire class) should now make a recommendation for which chemical reaction (and in what concentration) they would choose for an instant cold pack. They should justify their choice based on the results of their experiment and their criteria/constraints.

Assess

• Use the student worksheets to assess their performance in this lesson.

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Chemical Engineer

Add Favorite Remove Favorite More Menu

• Read More

Career Profile

Chemical engineers design the chemical reactions that go into making and using many of the products we use every day, from gasoline that fuels our cars to soap and shampoo you use in the shower. If you enjoyed exploring different chemical reactions in this experiment, you should read more about chemical engineering. Read more

Commercial & Industrial Designer

Add Favorite Remove Favorite More Menu

• Read More

Career Profile

Commercial and industrial designers design many products that people use. They think carefully about what the products look like so they will appeal to consumers. Read more

Lesson Plan Variations

• Have students test other exothermic reactions and choose the best one for a hot pack. Examples include the elephant toothpaste reaction or steel wool and vinegar reaction.
• Have students analyze the rate of temperature change for each reaction. For example, they can record the temperature every 10 seconds, and then make a graph of temperature vs. time. Try this with and without stirring. How does stirring affect the rate of temperature change?